Of all of the questions that I am asked, this is probably the most asked question. Unfortunately, it is the one with the least straight answer. From my research into the subject (and personal experience) there are several possible reasons this could be happening:

Your liners could be overfilled. With some recipes if you fill the liners too high instead of getting a pretty dome, the cupcakes rise and then collapse in the center because the cake can’t hold up the weight of itself. If you are not sure how high to fill the liners (see my post to how high to fill cupcake liners).

Your oven temperature could be off. Buy an oven thermometer and check to make sure you are baking at the correct temperature.

You don’t have patience. I know it’s hard to not peek at your beauties while they are baking, but try to avoid opening the oven. Every time you do it affects the oven temperature, which affects the end result of your baked good.

You mixed too much. Unless the recipe states otherwise, always mix only until the ingredients are just combined. In addition to causing the cupcakes to sink, overmixing can also make them taste gummy.

You are baking at great heights. If you live in a high altitude location, you may need to adjust your recipe to accommodate. You can refer to the guide to high altitude baking from the author of Pie in the Sky for more information.

You really don’t have patience. Make sure your cupcakes are completely baked before removing them from the oven. If they are not, they may deflate just after you take them out.